1. Field of the Invention
The present invention relates to an adhesion accelerator for enhancing the adhesive strength between rubber and metal. In particular, this invention relates to an adhesion accelerator for enhancing the adhesive strength of rubber to steel cords. In addition, the present invention also relates to a rubber composition containing the above adhesion accelerator.
2. Description of the Prior Art
Heretofore, in order to improve the adhesive strength of natural or synthetic rubber to steel cords used as a reinforcement for a radial tire or a belt of a belt conveyer, there have been used, as an adhesion accelerator, cobalt-containing metal organic compounds such as cobalt stearate, cobalt naphthenate, cobalt-boron complex, etc. Although these cobalt-containing metal organic compounds have a high vulcanization accelerating ability, the presence of cobalt in rubber lowers the storage stability. Besides cobalt-containing organic compounds, some other metal organic compounds have also been investigated. In fact, many patents disclose the use of some specific metal organic compounds as adhesive strength improving agents. For instance, some organic acid cobalt salts are disclosed in the following patents. U.S. Pat. No. 1,919,718 discloses cobalt acetate and cobalt salts of low molecular fatty acids, particularly a cobalt stearate. U.S. Pat. No. 2,912,355 discloses cobalt oleate and cobalt citrate. U.K. patent 1,169,366 discloses cobalt linoleate and cobalt resinate. U.S. Pat. No. 4,076,668 discloses cobalt salts of naphthenic acid, octylic acid, tall oil acid, etc. Japanese Patent Application Laid-Open No. 61-60743 discloses cobalt propionate, calcium propionate, magnesium propionate, nickel propionate, zinc propionate, etc. Japanese Patent Application Laid-Open No. 6-65142 discloses a metallic soap having a structure of naphthalene or biphenyl. Japanese Patent Application Laid-Open No. 60-15444 and Japanese Patent Application Laid-open No. 60-158230 disclose cobalt borate and the like. Japanese Patent Application Laid-Open No. 60-199643 discloses a metal salt of thiosulfuric acid ester. Japanese Patent Application Laid-Open No. 59-207949 and Japanese Patent Application Laid-Open No. 6-329838 disclose a metallic soap having a specific ratio of cobalt to nickel, as an adhesion improving material. Japanese Patent Application Laid-Open No. 6-329840 discloses a metallic soap having a specific ratio of cobalt to zinc, as an adhesion improving material. Although the above substances have a good adhesive property in a specific aspect, they still fail to show a well-balanced adhesive strength as a whole.
It is an object of the present invention to provide an improved adhesion accelerator having an excellent adhesive property well comparable with or far superior to that of a cobalt organic compound and having an improved stability for storing. It is another object of the present invention to provide a rubber composition containing such an adhesion accelerator.
The present inventors have found that the purposes of the invention can be achieved by adding a nickel organic compound and a zinc organic compound, in place of cobalt organic compounds. Namely, the present inventors have found that adding a nickel organic compound and a zinc organic compound in combination can produce an excellent adhesive property as compared with the case using a cobalt organic compound alone, and that an adhesion accelerator containing the above two compounds can produce an excellent adhesive property during a process of vulcanization at a high temperature and provide an excellent storing stability to the rubber kneaded therewith.
Namely, the present invention is directed to:
(1) An adhesion accelerator for improving the bonding of vulcanized rubber to metal, in which the improvement is characterized in the adhesion accelerator comprising:
at least one nickel organic compound selected from the group consisting of Ni-containing soaps having carboxylic acid moieties each having 6 to 30 carbon atoms in its main chain, or a mixture thereof, an organic boron complex containing Ni bonded to a carboxylic acid moiety having 6 to 30 carbon atoms in its main chain, and Ni-acetyl acetonate, and
at least one zinc organic compound selected from the group consisting of Zn-containing soaps having carboxylic acid moieties each having 6 to 30 carbon atoms in its main chain, or a mixture thereof, an organic boron complex containing Zn bonded to a carboxylic acid moiety having 6 to 30 carbon atoms in its main chain, and Zn-acetyl acetonate,
the at least one nickel organic compound and the at least one zinc organic compound being blended together to have a Ni/Zn weight ratio of from 0.01 to 20 in the adhesion accelerator.
(2) The adhesion accelerator for bonding rubber to metal as described in the above (1), in which the nickel organic compound and the zinc organic compound are metallic soaps.
(3) The adhesion accelerator for bonding rubber to metal as described in the above (1), in which the weight ratio of Ni/Zn is from 0.05 to 10.
(4) A rubber composition for forming a vulcanized rubber having an improved adhesion to metal comprising a rubber component, sulfur and the adhesion accelerator as set forth in any of the above (1) to (3), the sulfur being present in an amount of 3 to 8 parts by weight and the adhesion accelerator being present in an amount of 0.1 to 10 parts by weight with respect to 100 parts by weight of the rubber component.
(5) A method of improving the bonding of vulcanized rubber to metal, in which the improvement is characterized in the adhesion accelerator being added to a rubber component and 3 to 8 parts by weight sulfur, based on 100 parts by weight of the rubber component, prior to the vulcanization thereof.
As set forth above, the present invention can provide synergistic improvements in the adhesive property and storage stability of a rubber composition due to the use of a combination of a nickel organic compound and a zinc organic compound.
The nickel organic compounds used in the present invention are as mentioned above and include nickel-containing compounds such as metallic soaps of a carboxylic acid. Examples of such compounds are nickel naphthenate, nickel neodecanate, nickel stearate, nickel-containing organic boron complex and nickel acetylacetonate.
The zinc organic compounds used in the present invention are as mentioned above and include zinc-containing compounds such as metallic soaps of a carboxylic acid. Examples of such compounds are zinc naphthenate, zinc neodecanate, zinc stearate, zinc-containing organic boron complex and zinc acetylacetonate.
Carboxylic acids used to form metallic soaps are natural or synthetic saturated or unsaturated carboxylic acids having 6 to 30 carbons in their main chains, or a mixture thereof. The carbon of a carboxylic group is excluded from the above-mentioned number of carbons. More specific examples are caproic acid, caprylic acid, capric acid, lauric acid, myristic acid, palmitic acid, stearic acid, iso-stearic acid, arachic acid, behenic acid, oleic acid, linoleic acid, linolenic acid, ricinoleic acid, 12-hydroxy stearic acid, dimeric acid, tall oil acid, naphthenate, neodecanoic acid, resin acid. Alternatively, there may be exemplified natural fatty acids, such as fish oil hardened fatty acid or beef tallow hardened fatty acid containing as its main component one or more of the above-listed acids. Therefore, as the metallic soaps obtained by using these carboxylic acids, nickel naphthenate and zinc neodecanate may be mentioned as preferable examples.
Further, in the present invention, metallic soaps as mentioned above may be used in the form of a mixed soap thereof and such a mixed soap may be prepared by a conventional process, such as a double decomposition process with alkali metallic soaps as intermediate products, a direct reaction process involving a direct reaction between metallic compounds and carboxylic acids, etc.
When a nickel organic compound and a zinc organic compound are to be added into a rubber material, these two compounds may be mixed in advance before being added into the rubber material, although they may be added separately to the rubber material.
Organic boron complexes containing Ni and Zn bonded to a carboxylic acid moiety having 6 to 30 carbon atoms in its main chain can be used as the nickel organic compound and the zinc organic compound, respectively. These organic boron complexes may be replaced by an organic boron complex containing both Ni and Zn. Examples of the organic boron complexes include an organic boron complex represented by the formula B(ONiOOCR)3-n(OZnOOCR)n, wherein R represents a hydrocarbon having 6 to 30 carbons and n is from 0 to 3. Carboxylic acids used to form the organic boron complexes may be those as mentioned above for the metallic soaps.
Further, the present invention also provides a rubber composition comprising rubber and the above-mentioned adhesion accelerator wherein the adhesion accelerator is added in an amount of 0.1-10 parts by weight with respect to 100 parts by weight of the rubber.
In the present invention, the mixing ratio of the nickel organic compound and the zinc organic compound is from 0.01 to 20, preferably from 0.05 to 10, more preferably from 0.1 to 5, in terms of the weight ratio of metal nickel to metal zinc, Ni/Zn. If the metal weight ratio of Ni/Zn is less than 0.01, any significant improvement cannot be obtained in adhesive strength as compared with a case of using a cobalt soap alone, during vulcanization at high temperature. On the other hand, if the ratio of Ni/Zn is more than 20, the adhesiveness will decrease.
In forming a rubber composition according to the present invention, as the rubber component, there can be used a natural rubber, a synthetic isoprene rubber, and some diene rubbers such as a styrene diene rubber, a polybutadiene rubber, all of which may be used as a mixture thereof. Further, the rubber component is preferred to contain a natural rubber and/or a synthetic isoprene rubber in an amount of 50 wt % or more.
Moreover, in the rubber composition of the present invention, the aforesaid adhesion accelerator is added in an amount of 0.1-10.0 parts by weight, preferably 0.2-5.0 parts by weight, with respect to 100 parts by weight of the rubber component. If the adhesion accelerator is added in an amount of less than 0.1 parts by weight, any improvement cannot be obtained in the adhesive strength after vulcanization. On the other hand, if the adhesion accelerator is added in an amount of more than 10.0 parts by weight, the adhesive strength will decrease.
Further, in the rubber composition of the present invention, a sulfur is added in an amount of 3-8 parts by weight with respect to 100 parts by weight of the rubber component.
Moreover, in the rubber composition of the present invention, a vulcanization accelerator (e.g., sulfene amide, chiazole, thiuram) may be added in an amount of 0.2-5 parts by weight with respect to 100 parts by weight of the rubber component.
In addition, compounding ingredients commonly used in a rubber manufacturing industry may be added in any appropriate amounts to the rubber composition of the present invention. Examples of such compounding ingredients include pigments such as carbon black or silica, softeners, anti-aging agents (antioxidants) such as amine/ketone, diarylamine, etc.
Although metal to be bonded to the rubber composition of the present invention is not limited to specific kinds or shapes, steel cords are preferably used. In order to enhance the adhesion between the metal and the rubber, it is preferred to use metal plated with brass or zinc or metal plated with brass or zinc combined with nickel or cobalt. More preferably, metal to be bonded with the rubber is plated with copper. Also, there is not any limitation to the twisting structure of such metal material.
The present invention will be described in more detail below by giving some examples and comparative examples in the following.